Touchpads, also known as digital pads, are coordinate type pointing devices used to input coordinate type data to computer systems and computer-controlled devices. A touchpad is typically a pressure-sensitive bounded plane capable of detecting localized pressure at its surface. When a user touches the surface with a finger, stylus, or the like, the circuitry associated with the touchpad determines and reports to the attached computer system the coordinates of the location touched. In response, the computer performs the function, if any, associated with the location pressed.
Typically one or more regions of the touchpad are assigned to certain functions within the system. The user is made aware of what function is associated with each region by a template. A template is a sheet with a graphic design and is typically placed over and in contact with the touchpad surface. The graphic design typically maps out regions of the touchpad surface and the regions are typically labeled to provide a reminder to the user as to the functions associated with the various mapped regions.
A touchpad's sensor is the heart of the touchpad. Several types of touchpad sensors are known in the art, such as switch closure type sensors. For example, a keyboard sensor having matrix of membrane switches is disclosed in U.S. Pat. No. 4,736,190 to Florella. A touch by a finger on a key causes a closure event between a pair of conductors. As with other switch closure type sensors, the location of the touch is detected using a bank of digital signal drivers and a bank of digital signal receivers.
Other devices known in the art are very complex. For example, U.S. Pat. No. 4,529,959 to Ito et al. discloses a sensor for a coordinate type input device comprising a braided lattice of conductors embedded in a compressible open-cell insulating material sandwiched between two insulating layers coated with a resistive material. Another sensor is shown in U.S. Pat. No. 4,455,450 to Margolin. This sensor involves resistive sheets sealed into a "pillow" form.
Typically one digital signal driver per row and one digital signal receiver per column is used. In high-resolution devices, the large number of drivers required significantly adds to the cost of the scanning electronics and, therefore, the cost of the touchpad. For example, a touchpad that is 640 rows by 480 columns will require 480 drivers and 640 receivers for a total of 1120 (640+480=1120) individual device pins. An electrical device having 1120 pins can be prohibitively expensive using programmable logic devices (PLDs) or even an application specific integrated circuit (ASIC).
In addition, high-density devices can have a relatively slow response time because each driver is sequentially activated and each receiver must be checked for each driver. A matrix that is 640 rows by 480 columns can require as many as 307,200 individual scans, because each row is sequentially driven by the drivers and each column must be scanned for each driven row.
It is therefore desirable to provide a high-resolution touchpad sensor without the need for a correspondingly large number of drivers and receivers.
It is also desirable to provide a touchpad sensor suitable for simplified scanning with a corresponding increase in the scanning bandwidth.